The influence of water on the spectroscopic properties of a liposoluble magnesium porphyrin

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Abstract
Complexation of water by chlorophyll molecules in natural photosynthetic processes is a matter of great interest. In the present work the influence of water on a model porphyrin, magnesium 2,7,12,17-tetrahexyl-3,8,13,18-tetramethyl porphin (MgTHTMP) has been studied by infrared spectroscopy. A CCl4 (spectroscopic grade) solution of MgTHTMP showed two small bands in the 3600 cm−1 infrared region. These bands are assigned to ν3 and ν1 vibrations of water coordinated to the Mg atom. For MgTHTMP in the solid state, either deposited on an infrared plate or suspended in a Nujol film, none of the OH bands is observed. An elemental analysis confirmed that the solid porphyrin was anhydrous. This evidence indicates that while no water is complexed with MgTHTMP in the solid state, this porphyrin is highly hygroscopic in a CCl4 solution and will easily complex with the few water molecules (less than 0.01%) present in the spectro grade solvent by forming a coordinate bond between the central magnesium atom and the oxygen of water. Several other metallic porphyrins in CCl4 solutions have been studied for comparison with the Mg porphyrin. The spectra of these substances did not show any complexed water. A model of the H2O–MgTHTMP interaction is given and the implication of this model on the aggregation states of chlorophyll will be discussed.